Chap. Vn. SUMMARY ON SLEEP OF LEAVES. 407 



head. But we may recall the behaviour of Mimosa 

 in the North, where the sun does not set, and the 

 complete inversion of the daily movements by artificial 

 light and darkness. It has also been shown by us, 

 that although leaves subjected to darkness for a mode- 

 rately long time continue to circumnutate, yet the 

 periodicity of their movements is soon greatly dis- 

 turbed, or quite annulled. The presence of light or 

 its absence cannot be supposed to be the direct cause 

 of the movements, for these are wonderfully diversified 

 even with the leaflets of the same leaf, although all 

 have of course been similarly exposed. The move- 

 ments depend on innate causes, and are of an adaptive 

 nature. The alternations of light and darkness 

 merely give notice to the leaves that the period has 

 arrived for them to move in a certain manner. We 

 may infer from the fact of several plants (Tropseolum, 

 Lupinus, &c.) not sleeping unless they have been well 

 illuminated during the day, that it is not the actual 

 decrease of light in the evening, but the contrast 

 between the amount at this hour and during the early 

 part of the day, which excites the leaves to modify 

 their ordinary mode of circumnutation. 



As the leaves of most plants assume their proper 

 diurnal position in the morning, although light be 

 excluded, and as the leaves of some plants continue to 

 move in the normal manner in darkness during at 

 least a whole day, we may conclude that the periodi- 

 city of their movements is to a certain extent in- 

 herited.* The strength of such inheritance differs 



* Pfeffer denies such inherit- " Nacliwirkunp;," or ihe after- 



ance ; he attributes (' Die Period. effects of light and darkness. 



Bewegungen,' pp. 30-56) tlie But we are unable to follow his 



periodicity when prolonged for train of reasoning. There does 



a day or two in darkness, to not seem to be anymore reason foi 



